Method for reducing microcontaminants during synthesis of pentachlorophenol

ABSTRACT

A method for reducing contaminants during synthesis of pentachlorophenol includes providing a phenol-based starting material and a catalyst, which form a reaction mixture. A chlorine flow is introduced so that it is in contact with the reaction mixture, and the starting material and chlorine are reacted via a temperature-programmed reaction. The chlorine flow is terminated at a predetermined temperature prior to an end of the temperature-programmed reaction and/or at a point where the yield of pentachlorophenol is less than about 95%.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/388,816, filed Mar. 24, 2006 now U.S. Pat. No. 7,446,235, which isincorporated by reference herein in its entirety.

BACKGROUND

The present disclosure relates generally to synthesis ofpentachlorophenol, and more particularly to method(s) for reducingmicrocontaminants during synthesis of pentachlorophenol.

Pentachlorophenol is a wood preservative generally manufactured viacatalyzed chlorination of phenol or chlorophenol mixtures in the liquidphase. The process for synthesizing pentachlorophenol often results inthe production of ppm levels of polychlorinated dibenzodioxins (PCDD)and polychlorinated dibenzofurans (PCDF) with six or more chlorinesubstituents.

Generally, such processes have a large increase in the toxic equivalent(TEQ) concentration during a narrow window in time near the maximumpentachlorophenol yield. Further, the TEQ concentration seems toincrease further in the cooled and solidified post-reaction samples.

Reduction of the microcontaminant levels in the pentachlorophenolproduct is desirable for many reasons, and in particular, forenvironmental purposes. The toxins that may be produced during synthesisof the pentachlorophenol product may cause or increase the risk ofundesirable environmental and health effects.

As such, it would be desirable to provide a method for synthesizingpentachlorophenol with substantially reduced amounts ofmicrocontaminants.

SUMMARY

A method for reducing contaminants during synthesis of pentachlorophenolincludes providing a phenol-based starting material and a catalyst. Thephenol-based starting material and catalyst form a reaction mixture. Achlorine flow is introduced so that it is in contact with the reactionmixture, and the starting material and chlorine are reacted via atemperature-programmed reaction. The chlorine flow is terminated at apredetermined temperature prior to an end of the temperature-programmedreaction and/or at a point where the yield of pentachlorophenol is lessthan about 95%.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present disclosure will become apparentby reference to the following detailed description and drawings. In eachof the Figures, the TEQ level is given in ppm for selected points oneach plot.

FIG. 1 is a graph depicting the temporal variation of chlorophenolyields and toxic equivalent (TEQ) concentrations for synthesis in whichchlorine flow was reduced throughout the process;

FIGS. 2A-2B are graphs depicting the temporal variation of chlorophenolyields and TEQ concentrations for synthesis with chlorine flow stopped,respectively, at 174° C. and 186° C.;

FIG. 3A is a graph depicting temporal variation of chlorophenol yieldsand TEQ concentrations for synthesis with chlorine flow stopped at 180°C., and the addition of 0.75 g AlCl₃;

FIG. 3B is a graph depicting temporal variation of chlorophenol yieldsand TEQ concentrations for synthesis with chlorine flow stopped at 180°C., and the addition of 10 g tetrachlorophenol;

FIGS. 4A-4C are graphs depicting temporal variation of chlorophenolyields and TEQ concentrations for synthesis with chlorine flow stopped,respectively, at 177° C., 180° C., and 183° C., followed by nitrogenpurging;

FIG. 5 is a graph depicting temporal variation of chlorophenol yieldsand TEQ concentrations for synthesis with chlorine flow and temperatureramp stopped at 183° C., followed by nitrogen purging;

FIGS. 6A-6B are graphs depicting temporal variation of chlorophenolyields and TEQ concentrations for synthesis with chlorine flow stoppedrespectively at 180° C. and 183° C., followed by nitrogen purging andthe addition of phenol;

FIG. 7 is a graph depicting the average end-of-run TEQ concentrationsfrom experiments with different chlorine flow stopping temperatures;

FIG. 8 is a graph depicting temporal variation of chlorophenol yieldsand TEQ concentrations for synthesis starting with phenol, and withchlorine flow and temperature ramp stopped at 180° C., followed bynitrogen purging;

FIGS. 9A-9C are graphs depicting temporal variation of chlorophenolyields and TEQ concentrations for synthesis with chlorine flow stoppedat 180° C., and with 2500 ppm of nickel added at 180° C., 160° C., and90° C., respectively;

FIGS. 10A and 10B are graphs depicting temporal variation ofchlorophenol yields and TEQ concentrations for synthesis with chlorineflow stopped at 180° C., and 2500 ppm nickel added at 90° C.;

FIG. 10C is a graph depicting temporal variation of chlorophenol yieldsand TEQ concentrations for synthesis with chlorine flow and temperatureramp stopped at 180° C., and 2500 ppm nickel added at 90° C.;

FIGS. 11A and 11B are graphs depicting temporal variation ofchlorophenol yields and TEQ concentrations for synthesis with chlorineflow stopped at 180° C., 2500 ppm nickel added at 90° C., and theaddition of 0.75 wt. % AlCl₃ and 1.5 wt. % AlCl₃ respectively;

FIGS. 12A-12D are graphs depicting temporal variation of chlorophenolyields and TEQ concentrations for synthesis with chlorine flow stoppedat 180° C., and, respectively, 5000 ppm, 1250 ppm, 833 ppm, and 100 ppmnickel added at 90° C.; and

FIG. 13 is a graph depicting temporal variation of chlorophenol yieldsand TEQ concentrations for synthesis starting with phenol, with chlorineflow and temperature ramp stopped at 180° C., and 2500 ppm Ni added at90° C.

DETAILED DESCRIPTION

Embodiments of the method disclosed herein advantageously reduce theformation of toxic equivalent (TEQ) concentrations of PCDD and PCDFduring synthesis of pentachlorophenol. Without being bound to anytheory, it is believed that such a reduction may reduce the healthand/or environmental risks generally associated with pentachlorophenol.

An embodiment of the method includes reacting a phenol-based startingmaterial and a catalyst to form a reaction mixture. The reaction mixturemay be contacted with chlorine in a reactor so as to formpentachlorophenol via a temperature-programmed reaction. As referred toherein, the term “temperature-programmed reaction” means that thetemperature of the reaction mixture is controlled according to apredetermined course in which it varies over time. In other embodiments,the temperature of the reactor is gradually increased throughout thetemperature-programmed reaction. The reaction generally begins at themelting point of the starting material, and the reactor temperaturegenerally does not exceed 190° C.

As the reaction progresses, the reaction mixture may be substantiallycontinuously mixed. Stirring and other mixing techniques are well knownto those skilled in the art, and commercial mixers are readilyavailable.

Non-limitative examples of suitable phenol-based starting materialsinclude phenols, chlorophenols (a non-limitative example of whichincludes 2,4,6-trichlorophenol), and/or combinations thereof. It is tobe understood that any suitable amount of starting material may be used.Furthermore, the amount of starting material may depend on, at least inpart, the scale of the reaction.

Catalysts that are effective for catalyzing the chlorination of phenolsare generally known in the art, and it is to be understood that anysuitable catalyst may be selected for use in embodiment(s) of the methoddisclosed herein. In an embodiment, the catalyst is AlCl₃. Generally,the amount of the catalyst ranges from about 0.25 wt. % to about 1.5 wt.%. The amount of catalyst may vary depending, at least in part, on thescale of the reaction.

A chlorine flow/feed is initiated so that it contacts the reactionmixture at a predetermined time and/or at a predetermined temperature.In certain embodiments, the chlorine flow is initiated when the reactortemperature reaches about 105° C. In other embodiments, the chlorineflow is initiated as soon as the reaction mixture melts. The chlorineflow/feed may be introduced at any suitable flow rate.

The freezing point/melting point of the reaction mixture increases aschlorination proceeds; for this reason, the reactor temperature isgradually increased as the reaction progresses, so as to substantiallymaintain a liquid phase of the reaction mixture (i.e., above the meltingpoint). In preferred embodiments, the temperature is maintained at lessthan about 5 degrees, more preferably less than about 2 degrees, andstill more preferably less than about 0.5 degrees above the meltingpoint of the reaction mixture.

As the synthesis of pentachlorophenol progresses, other chlorinatedphenols may form, and then be converted or consumed, or remain in thefinal product. Examples of other chlorinated phenols that may formduring the synthesis of pentachlorophenol include 2-chlorophenol,4-chlorophenol, 2,4-chlorophenol, 2,6-di-chlorophenol,2,4,6-tri-chlorophenol, and/or tetrachlorophenol.

Embodiments of the method include terminating the chlorine flow at apredetermined temperature prior to an end of the temperature-programmedreaction. It has been discovered that terminating the chlorine flowprior to the end of the synthesis substantially reduces TEQconcentrations in both the pentachlorophenol product from the reactorand the post-reaction pentachlorophenol product. It has also beendiscovered that TEQ concentrations are further reduced when sometetrachlorophenol remains unconverted.

Thus, according to preferred embodiments, the chlorination flow isterminated when the reaction mixtures reaches a temperature ranging fromabout 170° C. to about 190° C. In specific non-limitative examples, thechlorine flow is terminated at 174° C., 177° C., 180° C., 183° C. or186° C. By way of example only, where the phenol-based starting materialis 2,4,6-trichlorophenol and the chlorine flow is terminated at atemperature between about 170° C. to about 190° C., the toxic equivalentconcentration of microcontaminants in a post-reaction sample ofpentachlorophenol is less than about 0.85 ppm. By way of furtherexample, where the phenol-based starting material is phenol and thechlorine flow is terminated at a temperature between about 170° C. toabout 190° C., the toxic equivalent concentration of microcontaminantsin a post-reaction sample of pentachlorophenol is less than about 2.4ppm.

Prior to termination, the chlorine flow may be gradually reduced as thetemperature is raised. The initial/first chlorine flow rate may bereduced to a second flow rate after a first predetermined time lapses ora first predetermined temperature is reached, and then the second flowrate may be reduced to a third flow rate after a second predeterminedtime lapses or a second predetermined temperature is reached. It is tobe understood that the flow rate may be reduced continuously or as manytimes as desirable prior to termination of the flow. Furthermore, thetime(s) at which the flow rate is reduced may be at any suitabletime(s), depending, at least in part, on the progression of thesynthesis.

In other embodiments, the chlorine flow is terminated at a point wherethe yield of pentachlorophenol (based on the amount of startingmaterial) is less than about 95%. By way of example, the chlorine flowmay be terminated at a point where the pentachlorophenol yield is lessthan about 80%, or less than about 85%. In an alternate embodiment, thechlorine flow may be terminated at a point where the tetrachlorophenolyield remains greater than about 1%, more preferably between about 1%and about 15%, and still more preferably between about 2% and about 10%.

The method may also include adding a metal to the reaction mixture priorto terminating the chlorine flow. Without being bound to any theory, itis believed that the addition of the metal may further reduce the TEQconcentration of microcontaminants in the pentachlorophenol.Non-limitative examples of the metal include nickel, cobalt, manganese,and combinations thereof. Generally, the metal may be in any suitableform, including powder form.

The metal may be introduced in an amount so that its concentration inthe reaction mixture is from about 100 ppm to about 5,000 ppm.Alternately, the metal may be introduced in an amount so that itsconcentration in the reaction mixture is from about 800 ppm to about5,000 ppm. Some specific non-limitative examples of the amount of metaladded include 833 ppm, 1,250 ppm, and 2,500 ppm.

The metal may be added to the reaction mixture prior to initiating thechlorine flow (e.g., when reactor temperature is about 90° C.), or priorto terminating the chlorine flow (e.g., when the reactor temperature isabout 160° C.). It is believed that adding the metal when or after thechlorine flow is terminated is generally not as effective in inhibitingthe formation of TEQ concentrations.

In a non-limitative example in which the phenol-based starting materialis 2,4,6-trichlorophenol, the chlorine flow rate is stopped at atemperature of about 180° C., and nickel is added prior to chlorine flowtermination, the toxic equivalent concentration of contaminants in apost-reaction sample of pentachlorophenol is less than about 0.5 ppm. Inan alternate non-limitative example in which the phenol-based startingmaterial is phenol, the chlorine flow rate is stopped at a temperatureof about 180° C., and nickel is added prior to chlorine flowtermination, the toxic equivalent concentration of contaminants in apost-reaction sample of pentachlorophenol is generally not greater thanabout 1.1 ppm.

In certain embodiments, upon termination of the chlorine flow, an inertgas flow may be introduced into the reaction mixture. A non-limitativeexample of such an inert gas is nitrogen gas (N₂). It is to beunderstood that the inert gas flow may also be used in embodiments ofthe method with or without the addition of metal.

In any of the embodiments of the method disclosed herein, additionalcatalyst material or tetrachlorophenol may be added after the chlorineflow is terminated. While the experimental details below indicate thatsuch an addition may have no appreciable effect on the TEQconcentrations, it is believed that an aluminum chloride catalyst mayaccelerate consumption of any residual chlorine, and/or thattetrachlorophenol may provide additional material with which theresidual chloride may react, thereby reducing the formation ofcontaminants.

In still other embodiments of the method, the temperature ramp may beterminated at the same time that the chlorine flow is terminated, and/orchlorine sponges may be added to the reaction mixture after terminationof the chlorine flow. Without being bound to any theory, it is believedthat altering the method by incorporating different combinations of thepreviously described steps may further reduce the TEQ concentrationsduring the synthesis of pentachlorophenol when the chlorine flow rate isterminated prior to the end of the temperature-programmed reaction.

To further illustrate embodiment(s) of the present disclosure, variousexamples are given herein. It is to be understood that these examplesare provided for illustrative purposes and are not to be construed aslimiting the scope of the disclosed embodiment(s).

EXAMPLES

The following experiments were performed in a laboratory. As such, thespecifics (i.e., quantities, stir rates, flow rates, etc.) apply to alab-scale synthesis. It is to be understood that quantities, stir rates,flow rates, etc. will be larger when synthesis is performed on acommercial scale.

Experimental Details—Chlorine Flow Rate Termination

For most of the following experiments, chlorine gas was bubbledcontinuously through a liquid phase containing, initially, about 300 gof molten 2,4,6-trichlorophenol and about 0.75 g AlCl₃ catalyst. Thepentachlorophenol synthesis was performed via a temperature-programmedreaction. The reactor temperature was gradually increased from about105° C., the temperature at which the chlorine flow/feed was initiated,to at greatest 190° C. during the synthesis to keep the contents in theliquid phase at all times. The freezing point/melting point of themixture increases as chlorination proceeds, so the reactor temperaturewas increased to remain just above the freezing point.

Samples were withdrawn periodically by inserting a glass rod into thereactor. The liquid on the rod quickly solidified when removed from thereactor and cooled to room temperature. At the end of each synthesis,the reactor contents were poured into disposable aluminum pans to cool.Samples of the post-reaction solid products may also be collected andanalyzed.

The samples were analyzed by capillary column GC to determine theamounts of tri-, tetra-, and pentachlorophenol. The microcontaminantlevels were determined by an aryl hydrocarbon receptor capture (AhRC)method that uses real-time polymerase chain reaction (PCR) forquantification. This AhRC PCR bioassay provides the TEQ concentration ofPCDDs and PCDFs in each sample.

Base Case

All samples were analyzed from a base-case synthesis experiment. Theresults from later experiments may be compared with the results of thebase-case.

In the base-case synthesis, about 0.25 wt. % (about 0.75 g) AlCl₃catalyst was added to molten trichlorophenol at 90° C. Chlorine flow wasintroduced at a rate of about 1.5 mol/hr, which was reduced to about 1.1mol/hr after about 2.5 hours, and was further reduced to about 0.9mol/hour after a 30 minute temperature ramp. The stirring rate was setat about 100-120 rpm.

FIG. 1 shows that the TEQ concentration is low initially, but increasesas tetrachlorophenol forms. As depicted, the TEQ concentration is lessthan about 0.5 ppm as long as tetrachlorophenol is still present in thesample. After the tetrachlorophenol is consumed, however, the TEQconcentration increases rapidly from about 0.56 to about 2.9 ppm. TheTEQ concentration continues to increase, to about 5.5 ppm, in thepost-reaction sample (the TEQ concentration for the post-reaction sampletaken from the aluminum pans is referred to as “Pan” in applicableFigures).

These observations suggest that product with TEQ concentrations around0.5 ppm may be obtained by stopping the synthesis while sometetrachlorophenol remains in the sample.

Termination of Chlorine Flow

FIG. 2A depicts results from experiments where the chlorine flow wasstopped at 189 minutes and where the reaction mixture had reached 174°C. The maximum pentachlorophenol yield, as shown in FIG. 2A, was about80% at 201 minutes, with about 0.27 ppm TEQ concentration. Thepost-reaction sample had a pentachlorophenol yield of about 77% withabout 0.3 ppm TEQ concentration. The final product contained about 15%tetrachlorophenol.

FIG. 2B depicts results from experiments where the chlorine flow wasstopped at 266 minutes and where the reactor temperature was 186° C.When comparing FIGS. 2A and 2B, the maximum pentachlorophenol yield andthe TEQ concentration were higher in the sample of FIG. 2B than in thesample of FIG. 2A. FIG. 2B shows that the pentachlorophenol yield wasabout 97% and the TEQ concentration was about 0.51 ppm when the chlorineflow was terminated. The TEQ concentration increased to about 0.81 ppmafter about 5 additional minutes of reaction time. The post-reactionsample had a pentachlorophenol yield of about 97% with a 0.81 ppm TEQconcentration. The final product contained no residualtetrachlorophenol, as this intermediate product had been consumedcompletely. It is apparent that the TEQ concentrations are lower in FIG.2A, where some tetrachlorophenol remains unconverted, than in FIG. 2B,where it had all been consumed. Thus, the precise temperature at whichthe chlorine flow is terminated may alter the TEQ concentration.

In comparing both FIGS. 2A and 2B to FIG. 1, the TEQ concentrations inthe post-reaction samples of FIGS. 2A and 2B were lower than those fromthe base-case synthesis run (FIG. 1). Without being bound to any theory,it is believed that stopping the chlorine flow near, but prior to, theend of the synthesis run is an effective way to stop the formation ofhigher TEQ concentrations. Still further, the results of FIGS. 2A and 2Bindicate that increases in TEQ concentration that occur post-reaction(as the molten pentachlorophenol cools) may be substantially inhibitedby stopping the chlorine flow in the reactor prior to the end of therun.

Addition of AlCl₃ or Tetrachlorophenol

FIGS. 3A and 3B illustrate data from synthesis experiments where thechlorine flow was stopped at 180° C., and at the same time about 0.75 gof AlCl₃ catalyst (FIG. 3A) or 10 g of tetrachlorophenol (FIG. 3B) wasadded.

In FIG. 3A, with added AlCl₃, the post-reaction sample (pan) had apentachlorophenol yield of about 87% with about 0.66 ppm TEQconcentration. In FIG. 3B, with added tetrachlorophenol, thepost-reaction sample (pan) had about 91% pentachlorophenol with about0.8 ppm TEQ concentration. In both examples, the residualtetrachlorophenol yield was about 6% when the chlorine flow stopped.

The results shown in both FIGS. 3A and 3B indicate that the TEQconcentrations are no lower than those in FIGS. 2A and 2B at equivalentpoints during the reactions. As such, it is believed that neither theaddition of AlCl₃ nor of tetrachlorophenol near the end of the synthesisresults in a substantial reduction or a substantial increase in theconcentration of microcontaminants.

FIGS. 2A, 2B, 3A and 3B show that the TEQ concentrations at the maximumtetrachlorophenol yields were typically lower than those of the finalpentachlorophenol sample in the aluminum pan. On average, abouttwo-thirds of the final TEQ value has developed by the time the maximumtetrachlorophenol yield is attained.

Purging with Nitrogen and Stopping Temperature Program

The previously described syntheses of pentachlorophenol likely haveresidual chlorine present at the end of the reactions. As the potentialexists for the residual chlorine to affect the TEQ concentrations insubsequent samples, this experiment focused on removing as much of theresidual chlorine as possible. This was accomplished by bubbling aninert gas (N₂) through the reaction mixture after stopping the chlorineflow.

The effect of inert gas introduction was tested by performing three moresyntheses and stopping the chlorine flow at different temperatures nearthe end of the run (177° C. (FIG. 4A), 180° C. (FIG. 4B) and 183° C.(FIG. 4C)). 0.25 wt. % catalyst and a catalyst addition procedure (addedat 90° C.) were used for these experiments.

The results in FIGS. 4A, 4B and 4C are similar to those obtained fromanalogous experiments that terminated chlorine flow at slightlydifferent temperatures and without a nitrogen purge (see FIG. 2). InFIGS. 4A-4C, the TEQ concentration near the end of the run was about0.65±0.2 ppm, and it was occasionally more than 50% higher than the TEQvalue that had been reached at the maximum tetrachlorophenol yield.These results suggest that purging with nitrogen provides littleadditional benefit in terms of reducing the TEQ concentration in thepost-reaction product. Without being bound to any theory, however, it isbelieved that such purging may be beneficial at a commercial scale(i.e., where volumes are larger and the gas-liquid surface area/reactorvolume is smaller).

It is noted that FIG. 4B illustrates TEQ concentrations higher than anyof the TEQ concentrations in FIGS. 4A and 4C. As all the values in FIG.4B are high and inconsistent with the results of FIGS. 4A and 4C, it isbelieved that the results are spurious and may not indicate a genuinedifference with the other experiments.

In the experiments involving nitrogen purging, the reactor temperaturecontinued to increase to the same final value (188° C.) even after thechlorine flow was stopped at some predetermined lower temperature.Another experiment was run to determine whether the continuedtemperature ramp had any effect on the TEQ concentration. In thisexperiment, a reaction was run where both the temperature ramp andchlorine flow were stopped together at 183° C. FIG. 5 illustrates theresults from this experiment. When FIG. 4C is compared to FIG. 5, it isevident that neither the TEQ concentration nor the pentachlorophenolyield were substantially altered by stopping the temperature ramp.

Adding Phenol with Nitrogen Purge

Another method for potentially ridding the reaction mixture of anyresidual chlorine is to add phenol to serve as a “chlorine sponge” atthe reaction stopping point. FIGS. 6A and 6B depict the results of twosynthesis experiments where about 10 g of phenol were added at differentchlorine flow stopping points (180° C. and 183° C., respectively). Theresults may be compared with the results in FIGS. 4A-4C. The experimentswere identical except for the addition of phenol at the time whenchlorine flow stopped.

Comparing FIGS. 6A and 4B, the conclusion may be drawn that the additionof phenol at 180° C. reduces the TEQ concentration by about a factor ofthree to four. This effect may not be real, however, because the TEQconcentrations in FIG. 4B may be too high, as discussed earlier. Acomparison of FIGS. 6B and 4C shows that the TEQ concentrations arehigher in the run with added phenol. It is to be understood that thisdifference is small, and within the experimental uncertainty. It appearsthat the addition of phenol at 183° C. had no appreciable effect on theTEQ concentration. The lack of an effect at 183° C. also supports thenotion that the TEQ concentrations in FIG. 4B are high. Given theseresults, the addition of phenol to the reaction mixture near the end ofthe run does not substantially change the TEQ concentration.

FIG. 7 illustrates the TEQ concentrations obtained by stopping thechlorine flow rate before the end of the synthesis experiment.Specifically, FIG. 7 shows the average of the TEQ concentrations in thefinal reactor sample and the post-reaction sample for the base-caseexperiment and for experiments with chlorine flow stopped at differenttemperatures. All of the previously discussed data for each specificchlorine flow stopping temperature were averaged to compute the TEQconcentrations shown in FIG. 7. For example, the data in FIGS. 3, 4B,and 6A were used to calculate the mean TEQ concentration for stoppingchlorine flow at 180° C. Implicit in this calculation, then, is thenotion that the addition of “chlorine sponges”, purging with N₂, andstopping the temperature ramp all had much less influence on TEQconcentration than did stopping the chlorine flow rate.

The data of FIG. 7 shows that terminating the chlorine flow rate at 186°C. substantially prevents the formation of microcontaminants thatotherwise may have been present in the product. The TEQ concentrationappears to decrease further as the temperature at which chlorine flow isstopped is reduced. It is to be understood that the composition of thefinal product at the different temperatures is different than the oneproduced in the base-case in that it contains increasing amounts oftetrachlorophenol.

Synthesis Starting with Phenol

Pentachlorophenol was synthesized with phenol as the starting materialto determine whether using a starting material other than2,4,6-trichlorophenol produced any differences in the results.

About 143 g of phenol (an amount that should lead to an equivalentamount of pentachlorophenol product, by moles, obtained when about 300 gof 2,4,6-trichlorophenol is used) were added to a reactor at 60° C. with0.75 g of AlCl₃ (about 0.25 wt. % based on trichlorophenol). The samestirring rate (100-120 RPM) and chlorine flow rate (1.5 mol/hr) asexperiments that started with trichlorophenol were used.

The temperature in the reactor increased quickly to above 100° C. afterthe chlorine flow began, because of the exothermic reaction. Sampleswere collected about every 30 minutes. The reaction required more than10 hours to reach the desired end point. The chlorine flow rate andtemperature ramp were stopped at 180° C., so that some tetrachlorophenolwas left in the final product.

The chlorinated phenols that formed were 2-, and 4-chlorophenol, 2,4-and 2,6-di-, 2,4,6-tri-, tetra-, and pentachlorophenol. The yields ofthe chlorinated phenols were analyzed by gas chromatography with thermalconductivity detection. The chlorophenol yields, TEQ concentrations forselected samples, and the reactor temperature profile (illustrated bythe Δ plot line) appear in FIG. 8. The yields of the two mono- and twodichlorophenols were summed in the plot. The mono-, di-, andtrichlorophenol yields increased to maximum values of about 84% at 60minutes, about 83% at 151 minutes and about 92% at 241 minutes,respectively, and then decreased to zero at the time the next productreached its maximum yield (i.e., the yield of monochlorophenol was zeroat the time the dichlorophenol yields reached a maximum). The yield oftetrachlorophenol increased to its maximum value of about 74% at 464minutes. The amount of tetrachlorophenol remaining at the end of the runwas about 8%, and the pentachlorophenol yield was about 83% in thissample.

The TEQ concentrations were about 0.13, about 0.11, and about 0.14 ppmat the points of the maximum mono-, di-, and trichlorophenol yields.These concentrations should be considered estimates because the bioassayused was calibrated with pentachlorophenol samples. A different chemicalmatrix existed in these samples with the less chlorinated phenols.Nevertheless, the results indicate that the TEQ values were very lowuntil tetrachlorophenol was formed. The TEQ concentration increased toabout 1.98 ppm at the maximum tetrachlorophenol yield, and it was about1.62, about 2.44, and about 1.37 ppm in the last two pentachlorophenolsamples taken from the reactor and the pentachlorophenol sample in thealuminum pan, respectively. The TEQ concentration did not changeappreciably after the maximum tetrachlorophenol yield appeared. Rather,it was consistently about 1.9±0.5 ppm.

This TEQ concentration of about 2 ppm is higher that those obtained fromsyntheses starting with trichlorophenol. However, these results arestill less than the 5.5 ppm in the pan sample of FIG. 1. The higher TEQconcentration may be a result of the experiment with phenol lastingabout 200 minutes longer to go from tri- to pentachlorophenol than didthe runs started with trichlorophenol. The longer reaction time may haveprovided greater opportunity for the formation of microcontaminants.

Experimental Details—Chlorine Flow Rate Termination and Addition ofMetal

For the following experiments, pentachlorophenol was formed by theAlCl₃-catalyzed (0.25 wt %) chlorination of trichlorophenol in a roundbottom flask. Chlorine gas continuously bubbled through the moltenreaction medium. As chlorination occurred, the melting point of themixture increased, so the reactor temperature was increased to keep it afew degrees above the melting point. Samples of the reaction mixturewere withdrawn periodically and analyzed for chlorophenols (by gaschromatography) and for microcontaminants (by bioassay). At the end ofthe experiments, the reactor contents are poured into a disposablealuminum pan to cool and solidify. This post-reaction product was oftenanalyzed. Acetone, aqueous HNO₃ solution, and methanol were usedsuccessively to clean the reactor after each experiment.

The chlorine flow in all of the experiments was terminated when thereactor temperature reached 180° C. This operation mode leaves a smallamount of tetrachlorophenol unconverted in the reactor.

Varying Time of Nickel Addition

Three experiments were performed in which the time at which nickelpowder (about 0.75 g ˜2500 ppm) was added was the variable. In allcases, the chlorine flow to the reactor was replaced with N₂ when thereactor temperature reached 180° C.

The results shown in FIG. 9A may be compared with the other experimentsterminating chlorine flow at 180° C. The conditions of the experimentsare similar except for the addition of Ni powder at 180° C. Results fromseven different experiments that stopped chlorine flow at 180±3° C. wereused to calculate mean TEQ concentrations of 0.38±0.12 ppm for thesample at the maximum tetrachlorophenol yield, 0.53±0.14 ppm for thelast sample taken from the reactor, and 0.59±0.18 ppm for thepost-reaction sample from the aluminum pan. The uncertainties shown hereare the standard deviations about the mean.

The TEQ concentrations in FIG. 9A are slightly higher than the meansgiven above, but they are within the range of TEQ values seenexperimentally from experiments with chlorine flow stopping at about180° C. without added Ni. As such, the addition of Ni near the end ofthe run does not appear to have an effect on the TEQ concentrations.

The results shown in FIGS. 9B and 9C indicate that adding Ni earlier inthe synthesis does seem to have benefits. In FIG. 9B, the Ni was addedwhen the reactor reached about 160° C., and in FIG. 9C, the Ni was added(along with the catalyst) at 90° C. In both cases, the TEQconcentrations were around 0.3 ppm in all samples, and there was noincrease in TEQ concentration after the maximum tetrachlorophenol yieldwas reached.

Collectively these results suggest that Ni powder does not substantiallyalter the TEQ concentration present at the maximum tetrachlorophenolyield, and that it does not substantially reduce the TEQ concentrationmuch when added near the end of the run. Rather, the results indicatethat nickel inhibits the TEQ concentration increase that had occurred inother runs between the maximum tetrachlorophenol yield and the finalpentachlorophenol product.

Three additional experiments were performed to check the reproducibilityof these results. FIGS. 10A and 10B show the chlorophenol yields and TEQconcentrations obtained at the same experimental conditions used togenerate FIG. 9C. The TEQ values were about 0.38 ppm at the maximumtetrachlorophenol yield, and about 0.5 ppm for the last sample withdrawnfrom the reactor. The TEQ concentrations for the post-reactionpentachlorophenol samples taken from the aluminum pan were about 0.46ppm and about 0.65 ppm. FIG. 10C illustrates the results from anexperiment where the chlorine flow and the temperature ramp were stoppedat 180° C. The TEQ concentration was about 0.25 ppm for the last samplefrom the reactor, and about 0.34 ppm for the post-reactionpentachlorophenol product. These TEQ concentrations are similar to thoseshown in FIG. 9C, and thus confirm that the TEQ reductions obtained byadding Ni at the start of the synthesis reaction are reproducible.

The accuracy of the cleaning procedure was verified by periodicallydoing experiments with no added Ni and verifying that pentachlorophenolwith higher TEQ was produced. For example, one such run with no added Niled to a TEQ concentration at the maximum tetrachlorophenol yield of0.79 ppm and a TEQ concentration in the final post-reaction sample of2.4 ppm. These values are higher than those that appear for runs withadded Ni.

Additional Catalyst

As catalyst concentrations greater than 0.25 wt. % appeared to have amodest effect on reducing the TEQ concentration in the pentachlorophenolproduct (see FIG. 3A), experiments were conducted to test the behaviorof additional AlCl₃ catalyst (0.75 wt. % and 1.50 wt. %) when nickel wasalso added. In each of these experiments, the temperature ramp wasterminated at 180° C., where the chlorine flow was stopped.

The results from a run with the 0.25 wt. % catalyst concentrationappears in FIG. 10C. FIGS. 11A and 11B show the results for theexperiments with 0.75 wt. % catalyst and 1.5 wt. % catalyst,respectively. In spite of the different catalyst concentrations, thefinal samples taken from the reactor had similar TEQ concentrations(about 0.25, about 0.13, and about 0.23 ppm for 0.25, 0.75 and 1.5 wt. %catalyst, respectively). The TEQ concentrations in the post-reactionsamples taken from the aluminum pans were also similar (about 0.34,about 0.25, and about 0.38 ppm, for 0.25, 0.75 and 1.5 wt. % catalyst,respectively). The results indicate that running the synthesis atcatalyst concentrations higher than 0.25 wt. % has little or noappreciable effect on the TEQ concentrations when Ni is also added tothe reactor.

Varying Nickel Concentration

The effects of adding more and less nickel during the pentachlorophenolsynthesis were tested. FIG. 12A shows the results from a synthesisexperiment with 5000 ppm nickel added. The TEQ concentrations for thelast sample from the reactor and for the post-reaction sample were about0.26 ppm and about 0.44 ppm, respectively. These values do not differappreciably from the TEQ levels obtained with less Ni, which indicatesthat the higher Ni level did not appear to reduce the TEQ level anyfurther.

FIG. 12B shows the results from a synthesis using about 1250 ppm nickel.About 84% pentachlorophenol was produced at 225 minutes, with 12%tetrachlorophenol and about 0.33 ppm TEQ. About 90% pentachlorophenolwas obtained at 234 minutes and 180° C., with 5% tetrachlorophenolresidual and about 0.71 ppm TEQ. The pentachlorophenol yield in thepost-reaction sample was about 89% with 5% tetrachlorophenol and about0.37 ppm TEQ.

FIG. 12C shows the results from a synthesis using about 833 ppm nickel.In the last sample from the reactor (taken at 180° C. and 255 minutes),the pentachlorophenol yield was about 90%, with 7% tetrachlorophenolremaining and about 0.37 ppm TEQ concentration. The TEQ concentration inthe post-reaction pentachlorophenol sample, where the pentachlorophenolyield was about 87% with 6% tetra, was about 0.39 ppm.

FIG. 12D shows results from a synthesis using about 100 ppm nickel. TheTEQ concentrations were about 0.4 ppm. The results shown in FIGS.12B-12D indicate that using about 100 ppm, about 833 ppm, or about 1250ppm of nickel provides substantially similar TEQ concentrations (fromabout 0.3 to about 0.4 ppm) in the final pentachlorophenol product asusing 2500 ppm nickel.

Synthesis Starting with Phenol

About 143 g of phenol (instead of trichlorophenol) was used in thissynthesis. About 0.75 g of Ni powder was added to the reactor at 105° C.(300 minutes). A total of about 1.5 g of AlCl₃ was used, with 0.75 gadded at 60° C. and 0.75 g added at 105° C. The same stirring rate(100-120 RPM) and chlorine flow rate (1.5 mol/hr) used in experimentsthat started with trichlorophenol were used in this experiment. Sampleswere collected about every 30 min. The reaction required about 10 hoursto reach to a desired end point. The chlorine flow and temperature rampwere terminated at about 180° C., so that some tetrachlorophenolremained in the final product.

The chlorinated phenols that formed were 2-, and 4-chlorophenol, 2,4-and 2,6-dichlorophenol, 2,4,6-trichlorophenol, tetrachlorophenol, andpentachlorophenol. The yields were analyzed by gas chromatography withthermal conductivity detection. The chlorophenol yields, TEQconcentrations, and the reactor temperature (illustrated by the Δ plotline) are depicted in FIG. 13.

The yields of the two mono- and two dichlorophenols were summed in theplot. The mono-, di-, and trichlorophenol yields increased to maximumvalues of about 87% at 60 minutes, about 81% at 121 minutes, and about92% at 246 minutes, respectively, and then decreased to zero at the timethe next product reached its maximum yield (i.e., the monochlorophenolyield was zero at the time the dichlorophenol yield was at its maximum).The yield of tetrachlorophenol increased to its maximum of about 79% at434 minutes. About 12% tetrachlorophenol remained at 589 minutes, wherethe pentachlorophenol yield was about 79%. The TEQ concentrations wereabout 0.14, about 0.11, and about 0.13 ppm, for the samples with themaximum mono-, di-, and trichlorophenol yields. These quantitativeresults should be viewed as estimates, because the chemical matrix inthese samples differed from that in the more highly chlorinatedpentachlorophenol standards used to calibrate the bioassay.Nevertheless, the results indicate that the TEQ concentrations are verylow (˜0.1 ppm) until tetrachlorophenol was formed. The TEQ concentrationincreased to about 0.43 ppm at the maximum tetrachlorophenol yield. Itsubsequently increased further to about 0.79 ppm and about 1.10 ppm forthe last pentachlorophenol sample from the reactor and the post-reactionpentachlorophenol sample, respectively.

The results in FIG. 13 may be compared with results from a similarphenol chlorination experiment without added Ni (see FIG. 8). Theprevious experiments without added Ni led to a TEQ concentration nearthe end of the run of about 1.9±0.5 ppm. The end-of-run TEQconcentrations in FIG. 13 are only about half as large. This resultagain exhibits that adding Ni reduces the TEQ concentration in thepentachlorophenol product, in this instance for chlorination startingwith phenol.

Effect of Other Metals

Since it had been demonstrated that Ni powder has an effect on the TEQconcentration in pentachlorophenol, other metals were tested for similareffects. Pentachlorophenol was synthesized in the presence of the AlCl₃catalyst and about 2500 ppm of other transition metals (including Mn,Co, Mo, W, V, Ti, Cu, Fe, and Zn). These metals were added to thereactor as powders along with the AlCl₃ catalyst.

The temporal variation of the yields of chlorophenols were nearlyidentical in these experiments. One noticeable difference was for thesynthesis in the presence of Cu. This reaction took longer than theother experiments with added metal. The results indicated that thatcopper inhibits the chlorination reaction.

Table 1 summarizes the results for TEQ analyses from samples taken atcomparable reaction points with the different added metals. The firstrow shows the mean values (±std. dev.) from experiments in which thechlorine flow was terminated between 177° C. and 183° C., and no Ni wasadded. The second row shows the mean values (±std. dev.) for all runswith Ni added at the start of the reaction. The remaining rows showresults from single analyses of single samples from an individualexperimental run. The data in the first two rows illustrates that addingNi reduces the TEQ concentration in the post-reaction sample by aboutone third. The TEQ concentrations in row 2, with added Ni, are all aboutthe same, whereas the TEQ concentrations in row 1, without added Niappear to increase during the reaction. Thus, it appears that Niinhibits the formation of toxic microcontaminants that would otherwiseform after the maximum tetrachlorophenol yield has been reached.

TABLE 1 TEQ Concentrations (ppm) from Synthesis in the Presence of MetalPowders Sample Maximum Tetra- Next to Final Post- chlorophenol LastReactor Reactor Reaction Metal Yield Sample Sample Sample None 0.38 ±0.12 0.53 ± 0.14 0.59 ± 0.18 Ni (mean) 0.38 ± 0.14 0.32 ± 0.13 0.39 ±0.16 0.42 ± 0.11 Ni (FIG. 9C) 0.34 0.33 0.29 0.34 Ni (FIG. 10A) 0.390.07 0.53 0.46 Ni (FIG. 10B) 0.37 0.38 0.50 0.65 Ni (FIG. 10C) 0.28 0.510.25 0.34 Ni (FIG. 11A) 0.25 0.14 0.13 0.25 Ni (FIG. 11B) 0.33 0.42 0.230.38 Ni (FIG. 12A) 0.23 0.28 0.26 0.44 Ni (FIG. 12B 0.58 0.33 0.71 0.37Ni (FIG. 12C) 0.36 0.32 0.37 0.39 Ni (FIG. 12D) 0.35 0.30 0.43 0.40 Mn0.33 0.34 0.35 0.57 Co 0.18 0.25 0.29 0.35 Mo 0.40 0.20 0.64 0.42 W 0.310.24 0.55 0.90 V 0.74 0.45 0.69 0.84 Ti 0.49 0.60 0.62 1.13 Cu 1.01 0.670.88 0.79 Fe 0.34 0.51 0.90 0.93 Zn 0.15 0.43 0.46 0.80

In the experiments with added metal, the TEQ concentrations were belowabout 1 ppm in all but two sample. Some of the metals tested appear tobe modestly more effective at lowering TEQ than others. If one lists thefour metals that gave the lowest TEQ values for the last samplewithdrawn from the reactor and the four metals that gave the lowest TEQvalues for the sample taken from the post-reaction solidified product,Ni, Co, and Mn appear on both lists. The mean TEQ concentration in thesesamples for these three metals was about 0.39 ppm. Likewise, if onelists the four metals that gave the highest TEQ concentrations for bothsamples, Fe and V are appear on both lists. The mean TEQ concentrationfor these samples with these two metals was about 0.84 ppm.

Embodiments of the method disclosed herein advantageously reduce theamount of microcontaminants formed during the synthesis ofpentachlorophenol.

While several embodiments have been described in detail, it will beapparent to those skilled in the art that the disclosed embodiments maybe modified. Therefore, the foregoing description is to be consideredexemplary rather than limiting.

1. A method for reducing microcontaminants during synthesis ofpentachlorophenol, comprising: providing a phenol-based startingmaterial and a catalyst, the phenol-based starting material and catalystforming a reaction mixture; introducing a chlorine flow in contact withthe reaction mixture, and reacting the starting material and thechlorine via a temperature-programmed reaction; and terminating thechlorine flow at a predetermined temperature prior to an end of thetemperature-programmed reaction, thereby suppressing the formation ofmicrocontaminants in the pentachlorophenol.
 2. The method as defined inclaim 1 wherein the chlorine flow is introduced at a first flow rate,and the method further includes: reducing the first flow rate to asecond flow rate after a first predetermined time period lapses; andreducing the second flow rate to a third flow rate after a secondpredetermined time period lapses.
 3. The method as defined in claim 1wherein the phenol-based starting material is selected fromchlorophenols, phenols, and combinations thereof.
 4. The method asdefined in claim 3 wherein the phenol-based starting material is2,4,6-trichlorophenol, and wherein a toxic equivalent concentration ofcontaminants in a post-reaction sample of pentachlorophenol is less thanabout 0.85 ppm.
 5. The method as defined in claim 3 wherein thephenol-based starting material is phenol, and wherein a toxic equivalentconcentration of contaminants in a post-reaction sample ofpentachlorophenol is less than about 2.4 ppm.
 6. The method as definedin claim 1, further comprising adding to the reaction mixture a materialselected from nickel, cobalt, manganese, and combinations thereof priorto terminating the chlorine flow.
 7. The method as defined in claim 6wherein the material is added so that its concentration in the reactionmixture is at least about 100 ppm.
 8. The method as defined in claim 6wherein the material is added so that its concentration in the reactionmixture is from about 800 ppm to about 5000 ppm.
 9. The method asdefined in claim 6 wherein the phenol-based starting material is2,4,6-trichlorophenol, and wherein a toxic equivalent concentration ofcontaminants in a post-reaction sample of pentachlorophenol is less thanabout 0.5 ppm.
 10. The method as defined in claim 6 wherein the materialis nickel, the phenol-based starting material is phenol, and wherein atoxic equivalent concentration of contaminants in a post-reaction sampleof pentachlorophenol is not greater than about 1.1 ppm.
 11. The methodas defined in claim 1 wherein at least the chlorine flow introducingstep is carried out substantially in a liquid phase.
 12. The method asdefined in claim 1 wherein the predetermined temperature at which thechlorine flow is terminated ranges from about 170° C. to about 190° C.13. The method as defined in claim 1, further comprising purging thereaction mixture with nitrogen after terminating the chlorine flow. 14.The method as defined in claim 1, further comprising adding one of analuminum catalyst and tetrachlorophenol to the reaction mixture afterterminating the chlorine flow.
 15. A method for synthesizingpentachlorophenol, comprising: reacting a phenol-based starting materialand a catalyst via a temperature-programmed reaction, the phenol-basedstarting material and catalyst forming a reaction mixture; introducing achlorine flow in contact with the reaction material; and terminating thechlorine flow at a point where a yield of the pentachlorophenol based onthe amount of starting material is less than about 95%, therebysuppressing the formation of contaminants in the pentachlorophenol. 16.The method as defined in claim 15 wherein the phenol-based startingmaterial is selected from chlorophenols, phenols, and combinationsthereof.
 17. The method as defined in claim 15 wherein the phenol-basedstarting material is 2,4,6-trichlorophenol, and wherein a toxicequivalent concentration of contaminants in a post-reaction sample ofpentachlorophenol is less than about 0.85 ppm.
 18. The method as definedin claim 15 wherein the phenol-based starting material is phenol, andwherein a toxic equivalent concentration of contaminants in apost-reaction sample of pentachlorophenol is less than about 2.4 ppm.19. The method as defined in claim 15, further comprising adding to thereaction mixture a material selected from nickel, cobalt, manganese, andcombinations thereof prior to terminating the chlorine flow.
 20. Themethod as defined in claim 19 wherein the material is added so that itsconcentration in the reaction mixture is from about 100 ppm to about5000 ppm.
 21. The method as defined in claim 19 wherein the phenol-basedstarting material is 2,4,6-trichlorophenol, and wherein a toxicequivalent concentration of contaminants in a post-reaction sample ofpentachlorophenol is less than about 0.5 ppm.
 22. The method as definedin claim 19 wherein the material is nickel, the phenol-based startingmaterial is phenol, and wherein a toxic equivalent concentration ofcontaminants in a post-reaction sample of pentachlorophenol is notgreater than about 1.1 ppm.
 23. The method as defined in claim 15wherein at least the chlorine flow introducing step is carried outsubstantially in a liquid phase.
 24. The method as defined in claim 15wherein a predetermined temperature at which the chlorine flow isterminated ranges from about 170° C. to about 190° C.
 25. The method asdefined in claim 15 wherein the chlorine flow terminating step iscarried out at a point where a yield of tetrachlorophenol remainsgreater than about 2%.
 26. The method as defined in claim 15 wherein thechlorine flow terminating step is carried out at a point where the yieldof pentachlorophenol is less than about 85%.
 27. The method as definedin claim 15 wherein the chlorine flow terminating step is carried out ata point where the yield of pentachlorophenol is less than about 80%.